The disclosures herein relate generally to hip joint implants and more particularly to an extractor for removing an acetabular insert from a metal shell in a hip joint.
An acetabular shell and insert are mounted in the acetabulum for receiving a hip stem which is implanted in a femur. The hip stem terminates in a metal ball which rotatably fits into a socket in the acetabular insert. Originally, both the hip stem ball and the socket in the acetabular insert were each formed of metal. However, many problems arose from the original metal-to-metal contact due to the materials used, such that mechanical seizing, noise and other unacceptable conflicts occurred.
As a result, a plastic acetabular insert was developed which included a convex surface cemented directly into a reamed socket formed into the acetabulum or inserted into a metal shell. This solution avoided the troublesome metal-to-metal contact of the original metal ball seated in the metal socket, and provided a plastic socket formed in the insert for receiving the metal ball of the hip stem.
An improvement provided a titanium metal shell secured in the socket formed in the acetabulum. A threaded fastener or fasteners could be used to secure the shell to the acetabulum. A plastic modular insert is snapped into the metal shell. The insert includes a plastic socket formed therein for receiving the metal ball of the hip stem.
When it is necessary to remove the plastic insert from the metal shell due to wear of the socket, so that the plastic insert can be replaced, the insert could be pried loose or a screw could be threaded into the plastic insert. The screw threads through the plastic insert and bottoms out against the metal shell. Unable to penetrate the titanium metal shell, the plastic insert rides up on the screw threads and eventually snaps out of the metal shell. This extraction method has the benefit of withdrawing the plastic insert directly outwardly from the metal shell. This is beneficial because the metal shell in some cases, includes a porous convex surface to which the bone of the acetabulum attaches over time. Therefore, any prying or twisting forces applied to eject the plastic insert from the metal shell may actually have a detrimental effect on the attachment between the bone socket and the metal shell.
A problem with the metal ball in the plastic socket is that particles of the plastic (polyethylene) material flake off from wear contact with the metal ball. As these flakes collect, the body reacts to the flakes as foreign matter and as a result, adjacent bone material may eventually resorb or deteriorate away due to the presence of these plastic flakes.
In order to avoid this problem caused by the metal-to-plastic wear, a metal inlay has been inserted into the socket formed in the plastic insert. The metal used for the inlay is a cobalt chrome, which is the same metal as the ball of the hip stem. Thus, progress has come full circle, and once again a metal-to-metal contact exists between the hip stem ball and the insert socket. However, the improved materials and manufacturing methods avoid many of the problems associated with early metal-to-metal wear surfaces in such orthopedic implants.
Although wear is greatly reduced with the latest metal-to-metal ball and socket implants, a problem arises when it is necessary to replace the inlay-insert combination from the metal shell. Due to the presence of the metal inlay in the ball socket of the plastic insert, it is no longer possible to extract the insert from the metal shell by the above-mentioned screw extractor method. This is because the screw will not penetrate the cobalt chrome inlay and therefore, the screw is not accessible to thread through and eject the plastic insert straight out of the shell. As stated above, prying and twisting of the plastic insert is undesirable because of the possibility of damaging the connection between the bone and the abutting convex surface of the metal shell.
Therefore, what is needed is a method and apparatus for extracting the plastic insert from the metal shell without damaging the connection between the metal shell and the abutting bone socket.